Constancy PSY 310 Greg Francis Lecture 19 It s all an illusion! Brightness illusions Most people think of visual perception as a measurement of light As it reflects off of objects 1
Object identification A basic task of the visual system is to figure out whether changes in intensity are due to changes in objects (color, paint) or to changes in illumination (light, shadows, veils) One problem is that an edge from paint could be the same as an edge from a shadow Steps-movie.swf Ambiguity The process of identifying an pattern of light as a set of objects in inherently ambiguous There is no way that you can always be correct! The visual system must use some tricks to try to be right as often as possible We can expose those tricks with illusions 2
Snakes illusion The diamonds have the exact same scale of gray Snakes illusion We see a dark transparent veil in front of some rows Which means the paint of the diamond behind it must be lighter than the paint of the diamond without the veil 3
Shadows Shadows 4
Shadows Shadows 5
Shadows We interpret the shadow as producing a change in intensity Which means the object surfaces (paint) must be different There are lots of brightness illusions based on this idea It s a side effect of our effort to judge the properties of objects in different kinds of lighting We want to be able to identify an object s properties regardless of lighting Constancy (color, brightness) Let s look at one more Perceiving light 6
Perceiving light Perceiving light 7
Perceiving light Perceiving light 8
Perceiving light Demonstration Pick up a copy of the image and randomly select a set of instructions. Follow the instructions and come back to the classroom within 10 minutes 9
Light levels People can see objects across an amazing range of light levels Some is due to pupil size, rod vision Light levels Even within indoor lighting ranges, we see things in a wide variety of levels of light 10
Light levels Consider a photograph of an office The light from outside is so bright you cannot see anything The light on the floor is so dark you cannot see anything Light levels You can change exposure settings, but nothing really works well 11
Light levels You can change exposure settings, but nothing really works well You have to go through a complicated process to take a really good picture Lots of light (like a photographer studio) Or process and combine different pieces of information Photoshop Take the over-exposed image and exaggerate the shadows The makes the details in the dark places of the floor visible 12
Photoshop Take the under-exposed image and exaggerate the bright regions The makes the details in the bright places of the window visible Photoshop Combine everything in the right way, and you get a better picture Your brain is doing something similar, but in a much more sophisticated way 13
Constancy Conclusions We want to know the properties of objects Ambiguity It is not always possible to know if changes are due to object properties of illumination properties Visual system Uses clever tricks to choose Usually correct Sometimes easy to fool Next time Depth perception Monocular cues 14